How can I combine two render layers?
The setup using the alphaover node and transparent rendering is a good idea, try going back to that and make the following changes: First go to your Blur renderlayer (in the renderlayer panel) and turn on the Environment pass,

 

then go to the compositor and add a second alphaover node. Plug the output of the first alphaover node into the bottom input of the second, then plug the Environment pass from the renderlayer into the other image input and pipe the alphaover output to the composite node.